The present invention relates to a method and an arrangement for injection of fuel into a cylinder of a combustion engine, and to a combustion engine of the type known inter alia under the designation ATAC (Active Thermic Atmospheric Combustion) or HCCI (Homogeneous Charge Compression Ignition).
A problem with conventional diesel engines is that they produce high emissions of nitrogen oxides (NOx), as a result of very high combustion temperatures in limited portions of the cylinders. ATAC or HCCI engines are intended to avoid these emission problems and may popularly be described as a combustion of diesel engine and Otto engine. In their case, a premixed fuel/air mixture is introduced into the cylinder and is ignited by compression when the working piston is in the vicinity of its upper dead center position in the ignition phase. The advantages of ATAC engines include producing little or no NOx emissions and exhibiting a high degree of efficiency close to that of conventional diesel engines. ATAC engines avoid the problem described above partly by using a lean mixture resulting in lower combustion temperatures and partly because combustion is initiated substantially simultaneously within widespread regions of the combustion chamber. The total result is more even temperature distribution without very high combustion temperatures in portions of the combustion chamber.
However, achieving a relatively homogeneous premixed fuel/air mixture involves particular expedients in the engine and extra components in the intake system as compared with a conventional diesel engine, thereby making ATAC engines more expensive and more complicated. It has also been found to be difficult effectively to achieve homogeneous mixing, particularly when using conventional diesel fuel. The technique employed has in particular easily resulted in drops of fuel which are too large and settle on the walls of the combustion chamber causing bad fuel economy and high emissions of unburnt fuel.
One object of the present invention is to avoid the problem of the state of the art and to indicate a simpler and more economic and effective solution.
This is achieved with a method and an arrangement of the kind mentioned in the introduction. Air and fuel are mixed and ignited by compression. Substantially all of the fuel is injected into the engine cylinder in the region of the upper dead center point of the piston in the gas exchange stroke.
The result arrived at in a simple and advantageous manner is a homogeneous mixture of fuel and air, as the injection of the fuel takes place during a phase when hot residual gasses from previous combustion cycles are still in the cylinder, thereby promoting vaporization of the injected fuel. During the induction or intake stroke, the air drawn in (or fed in) becomes effectively mixed with the vaporized or at least substantially vaporized fuel so that a homogeneous fuel/air mixture is formed throughout the combustion chamber. After the induction stroke, compression takes place in a conventional manner, followed by ignition in the region of the upper dead center point in the compression stroke.
The engine is thus afforded advantages pertaining to homogenous combustion without having to adopt expedients which are necessary in ATAC engines. For example, no separate mixing chamber is required. On the contrary, the engine""s conventional injection system can be used with modified control of injection. More effective homogenization of the fuel/air mixture is also achieved. Injection control can be exercised by means of the engine""s computer system and injection can be physically effected by means of conventional mechanical, electrical, pneumatic or hydraulic devices intended for fuel injection in combustion engines.
According to an advantageous embodiment of the invention, all the fuel is injected into the cylinder in the region of the piston""s upper dead center point in the gas exchange stroke in the hot residual exhaust gases, which means that the fuel becomes properly mixed in the cylinder before being subjected to combustion.